Bobbin thread tensioning device for a sewing machine

Abstract

A device for automatically adjusting the tension of a lower thread, which comprises a lower thread bobbin arranged on a bottom plate of a bobbin carrier turnably carrying the bobbin, a loop taker mounted on a magnetic shaft, and an energizing coil arranged around the magnetic shaft and providing together with said bottom plate and magnetic shaft a path of magnetic flux. The device is further provided with a detector detecting the consumed amount of the bobbin thread and a control circuit for energizing the coil when the lower thread is wound to the maximum capacity of the bobbin, interrupting the light of the detector. The detector operates the control circuit to decrease the amount of electric current applied to the energizing coil so as to set the lower thread tension back to its initial predetermined value.

We claim: 
     
       1. A bobbin thread tensioning device for a sewing machine having a lower shaft, comprising a cup-shaped loop taker; a bobbin carrier accommodated within said loop taker and formed with a central bore; a magnetic bobbin for carrying a lower thread, said bobbin being received within said central bore and adapted to be rotated as the lower thread is drawn out; a magnetic central shaft rigidly secured to said loop taker and operatively connected to the lower shaft of the sewing machine for rotation therewith, said bobbin carrier being provided with a magnetic plate positioned in the vicinity with said magnetic shaft; detecting means for detecting the amount of the lower thread remaining on the bobbin during the operation of the sewing machine; and bobbin thread tension adjusting means including an energizing coil surrounding said magnetic shaft so that said magnetic bobbin, said magnetic plate and said magnetic shaft form a magnetic path passing through the center of said energizing coil, and a control circuit electrically connected to said energizing coil, said control circuit being operatively connected to said detecting means for adjusting the amount of the electric current flowing through said energizing coil and thus changing the electromagnetic force in said magnetic path and affecting the friction force between said magnetic plate and said bobbin thereby tightening the lower thread in response to the predetermined amount of the remaining lower thread detected by said detecting means. 
     
     
       2. The device of claim 1, wherein said detecting means include a luminous diode and a phototransistor, said diode and said phototransistor being positioned at the opposite sides of said bobbin carrier. 
     
     
       3. The device of claim 2, wherein said bobbin carrier is formed with holes provided against said diode and said phototransistor respectively to provide a passage for the light passing from said diode to said phototransistor. 
     
     
       4. The device of claim 3, wherein said bobbin carrier is made of a synthetic resin, said magnetic plate being located at the bottom of said bobbin carrier. 
     
     
       5. The device of claim 4, wherein said bobbin carrier is accommodated within said loop taker with a slight clearance therebetween. 
     
     
       6. The device of claim 5, further comprising means for preventing said bobbin carrier from rotation including two swingable arms adapted to hold said bobbin carrier when said magnetic central shaft with said loop taker are rotated.